These screws are used for mounting the Scorpion S-40mm airplane motors and HK-50mm series helicopter motors. Included in this kit are four 4x0.7x8mm socket head screws and four matching flat washers. The flat washers have an outside diameter of 8.8mm and are 0.75mm thick.
The backplate of the Scorpion motors are 4mm thick, so to get the right size screw, add 5mm to the thickness of your motor mount plate to get the proper length screw. This will allow for the thickness of the motor backplate and the washer.